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1 Copyright © 2009 American Water Works Association AWWA eLearning Water System Mechanical Equipment Module 3: Instrumentation and Control An operator’s main responsibilities are supervision and control. Supervision means examining system performance information and deciding if it is acceptable. If, in the operator’s opinion, performance is unacceptable, then an element of the system operation must be changed to bring performance back to an acceptable condition. A setup for which a human operator evaluates the performance continually is said to have open-loop control. Control equipment allows the operator to change valve settings, turn pumps on and off, and otherwise adjust the system for efficient operation. When instruments are provided to make the necessary change or correction automatically without the intervention of the operator, the system is said to have closed-loop control. However, regardless of the extent to which automatic control is used, the operator still may need to intervene manually during abnormal or emergency situations. Most large utilities now feed instrument readings to one or more computers both to gather information and to assist in providing control. Most small systems, though, continue to maintain simpler, on-site instru- mentation and controls. As computers and electronic control equipment become less expensive, less complicated, and more reliable, they will increasingly be used by medium-size and smaller utilities. The main categories of instrumentation and control discussed in this module are primary instrumentation: sensors that measure process variables such as flow, pressure, level, and temperature secondary instrumentation: instruments that respond to and display information from primary instrumentation control systems: manual, automatic, and digital systems that operate final control elements such as pumps and valves This module will also discuss maintenance and operation safety for this instrumentation.

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Page 1: Module 3: Instrumentation and Control 3: Instrumentation and Control ... • primary instrumentation: sensors that measure process variables such as flow, pressure, level, and temperature

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Copyright © 2009 American Water Works Association

AWWA eLearningWater System Mechanical Equipment

Module 3: Instrumentation and Control

An operator’s main responsibilities are supervision and control. Supervision means examining system performance information and deciding if it is acceptable. If, in the operator’s opinion, performance is unacceptable, then an element of the system operation must be changed to bring performance back to an acceptable condition. A setup for which a human operator evaluates the performance continually is said to have open-loop control. Control equipment allows the operator to change valve settings, turn pumps on and off, and otherwise adjust the system for efficient operation.

When instruments are provided to make the necessary change or correction automatically without the intervention of the operator, the system is said to have closed-loop control. However, regardless of the extent to which automatic control is used, the operator still may need to intervene manually during abnormal or emergency situations.

Most large utilities now feed instrument readings to one or more computers both to gather information and to assist in providing control. Most small systems, though, continue to maintain simpler, on-site instru-mentation and controls. As computers and electronic control equipment become less expensive, less complicated, and more reliable, they will increasingly be used by medium-size and smaller utilities.

The main categories of instrumentation and control discussed in this module are

• primary instrumentation: sensors that measure process variables such as flow, pressure, level, and temperature

• secondary instrumentation: instruments that respond to and display information from primary instrumentation

• control systems: manual, automatic, and digital systems that operate final control elements such as pumps and valves

This module will also discuss maintenance and operation safety for this instrumentation.

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Primary InstrumentationSensors that measure quantity of flow, pressure, and other parameters

are essential to water system operations. The measurements allow the operator to maintain the quality and quantity of drinking water efficiently.

Basic Instrument ComponentsVarious instruments are used to measure, display, and record the

conditions and changes in a water treatment plant and distribution system. Some instruments indicate what is happening at a given instant. Others guard against equipment overload and failure. Still others provide permanent records that are used to determine operating efficiency and the need for regularly scheduled maintenance.

These functions are similar to the functions of an automobile instru-ment panel. The speedometer indicates the current vehicle speed; the oil-pressure light warns of impending engine failure; and the mileage indicated on the odometer can be used to calculate gas mileage and deter-mine the need for regular maintenance. In both the water system and the automobile, the instruments do not replace the operator. They simplify the operator’s work and help to improve the performance, safety, and reliability of the equipment.

The simplest instruments have only two parts: a sensor and an indicator. The sensor responds to the physical condition (parameter) being measured, converting it to a signal that can activate the indicator. The signal may be a simple physical motion, or it may be an electrical current or a change in pneumatic (air) pressure.

The indicator may display the result immediately, or it may be replaced or supplemented by recorders or totalizers to monitor condi-tions over a period of time. The main categories of indicators are direct, remote, and distant. Indicators will be discussed in more detail later in this module.

The following descriptions introduce the most common types of sensors.

Flow SensorsThe most significant measurement in water treatment and distribution

facilities is the flow of water. Day-to-day operational decisions and long-term planning are based on the measurements from flowmeters. Knowledgeof the quantity of flow is needed to control plant inflow, pace chemical feeds, bill customers, check the efficiency of pumps, monitor for leaks, and help control or limit the delivery of water. Analyses of the water

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system operations are also derived from treatment plant flow and distri-bution system information.

The flow passing through treatment plant and distribution system meters might simply be totalized on the meter register. It can also be recorded locally on a chart recorder or transmitted to a central location for recording. It is also usually important to know the flow rate at various times of the day, as well as the flow in various parts of the distribution system. Figure 3-1 shows a chart recording of a water plant’s instanta-neous flow rate for a period of one week.

The following paragraphs review various meter technologies for mainline measurements.

FIGURE 3-1 Typical circular chart recording

Courtesy of Process Instruments, Inc.

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Differential-Pressure FlowmetersThe measurement devices most commonly used for measuring large

quantities of water are differential-pressure flowmeters (also called head meters). Their popularity is largely due to a combination of flexibility, simplicity, ease of installation, and reliability. The flow velocity reading of a differential-pressure meter is calculated from the difference between two pressures measured in the meter. Differential-pressure flowmeters are available in many forms, such as the venturi, modified venturi, flow tube, or orifice plate. The measurement of flow is a function of detecting two pressure heads, usually one in the normal pipe size and one in a constricted region, called the throat, within the meter. The flow is propor-tional to the square root of the difference between the two pressure readings.

Velocity-Type FlowmetersVelocity-type, or current-type, flowmeters include magnetic, turbine,

propeller, multijet, proportional, and sonic flowmeters. In each case, the flow velocity is measured, and the quantity is calculated from the product of velocity times the cross-sectional area of the pipe. The Pitot meter is also a velocity-type measuring meter, but it determines the velocity based on the difference between the flow’s dynamic pressure and the static pressure.

Other FlowmetersOther meters are available for mainline flow sensing, such as the

vortex- shedding flowmeter and the variable-area flowmeter. The open-channel flow detectors (weirs and flumes) represent another type of differential-pressure flowmeter. In both cases, the flow is calculated from the fluid depth or the head that drives the flow.

Pressure SensorsPressure sensors are used to determine suction and discharge pres-

sures at pumps, pressure regulators, and selected points in a distribution grid. They also determine pressures of plant waters, eductors, storage tanks, and air compressors. Pressure instrumentation may range from simple, direct-reading pressure gauges to complex pressure-sensing equipment that transmits readings to remote locations. The four most common types of pressure sensors are

• strain gauges

• bellows (low pressure)

• helical elements (medium pressure)

• Bourdon tubes (high pressure)

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Strain GaugesThe diaphragm strain gauge sensor is the type used most widely in

modern instrumentation. As shown in Figure 3-2A, it consists of a section of wire fastened to a diaphragm. As the variable being measured (such as pressure) changes, the diaphragm moves, changing the length of the wire, thus increasing or decreasing its resistance. This changing resistance can be measured and transmitted by electrical circuits. Similar electronic sensors include the variable-capacitance pressure cell, variable-reluctance pressure cell, and vibrating wires.

Direct-Reading Pressure GaugesThe remaining three types of sensors are direct-reading pressure

gauges, which were once widely used but are seldom installed today. The bellows sensor (Figure 3-2B) is a flexible copper can. The sensor expands and contracts with changes in pressure. The helical sensor (Figure 3-2C) is a spiral-wound tubular element that coils and uncoils with changes in pressure. The Bourdon tube (Figure 3-2D) is a semicircular tube with an elliptical cross section that tends to assume a circular cross-sectional shape with changes in pressure, thereby causing the C-shape to open up.

Level SensorsLevel sensors are commonly used to measure the elevation of water

in wells, as well as the depth of water in storage reservoirs and tanks. They are also used to measure the levels of stored chemicals in tanks. A pressure sensor can readily be adapted to level measurement if it is installed at the base of a tank. As level increases in the tank, the pressure reading increases. The reading can be calibrated in feet (or meters) of liquid. In elevated tanks, the level measurement needed is the level in the elevated portion of the tank, rather than in the tank and riser. Transmittingmechanisms can be calibrated so that “zero” represents the bottom or minimum level in the elevated portion of the storage tank.

The common types of level sensors are

• float mechanisms

• diaphragm elements

• bubbler tubes

• direct electronic sensors

Float MechanismsFloat mechanisms (Figure 3-3A) have a float that rides on the water

surface and drives the transducer through an arm or cable. Floats are very

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FIGURE 3-2 Types of pressure sensors

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inexpensive and simple in operation, but they cannot be used where the liquid surface may be rough or may freeze.

Where extreme accuracy in level-sensing systems is required, a special adaptation of a float-type level transmitter has been used called a stage recorder. It uses larger floats and a more precise pulley system to increase positioning accuracy. These devices may also be equipped with a digital sensing system.

Diaphragm ElementsDiaphragm elements (Figure 3-3B) have a flexible bulb or diaphragm

connected by a tube to a pressure sensor. They operate on the principle that the confined air in the tube will compress in relation to the head of water above the diaphragm. The change of pressure that is sensed is then related to a change in the head of water.

Bubbler TubesBubbler tubes (Figure 3-3C) maintain a constant low flow of air

discharging from the end of a tube suspended in the liquid. They operate on the principle that the pressure required to discharge air from the tube is proportional to the head of water above the tip of the tube. Both the diaphragm and the bubbler-tube sensors commonly use a strain gauge sensor to convert the changes in air pressure into an electrical signal.

Bubbler tubes were once widely used because they are almost maintenance-free and can be used in almost any type of liquid. They do require an air compressor to supply a continuous stream of air. Bubbler tubes have now generally been replaced with newer electronic equipment.

Direct Electronic SensorsIn recent years, direct electronic devices have been developed for

level measurement. Among these are various types of probes, variable-resistance devices, and ultrasonic systems.

In a probe system, an insulated metallic probe is installed in the reser-voir. As the water level rises and falls in the reservoir, the capacitance changes between the probe and the water. This capacitance signal can then be converted to a signal that indicates the level. When the probe is installed in a nonmetallic tank, a second electrode is required.

A variable-resistance level sensor consists of a wound resistor inside a semiflexible envelope. As the liquid level rises, the flexible outer portion of the sensor presses against the resistor and a portion of the resistor element is temporarily shorted out, which changes the resistance of the sensor. The resistance is then converted to a liquid-level output signal.

In ultrasonic level-sensing systems, an ultrasonic generator is installed above the water level. This generator sends ultrasonic signals

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toward the water surface, and the signals bounce back and are detected by a receiver. The time required for this signal to echo is calibrated to produce a water-level output signal. Air temperature variations must be compensated for because the speed of sound in air is a function of temperature. Excessive humidity in the air above the liquid may also significantly interfere with proper operation.

FIGURE 3-3 Types of level sensors

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Temperature SensorsTemperature sensors are commonly used in a water system to monitor

the condition of pump bearings, switchgear, motors, building heat, and water. The temperature sensors commonly used in distribution systems are thermocouples and thermistors.

Thermocouples use two wires of different materials, represented as metal A and metal B in Figure 3-4A. These wires are joined together at two points: the sensing point and the reference junction. Temperature changes between the two points cause a voltage to be generated, which can be read out directly or amplified through a transducer.

Thermistors (Figure 3-4B), also called resistance temperature devices (RTDs), use a semiconductive material, such as cobalt oxide, that is compressed into a desired shape from the powder form. The material is then heat-treated to form crystals to which wires are attached. Tempera-ture changes are reflected by a corresponding change in resistance of the thermistor, as measured through the attached wires.

Electrical and Equipment SensorsMeasurements of electric power and equipment status are important

for maintaining operational efficiency.

Electrical SensorsFour important electrical parameters are monitored for various pieces

of equipment in the distribution system:

• voltage, measured as volts

• current, measured in amperes

• resistance, measured in ohms

• power, measured in watts

FIGURE 3-4 Types of temperature sensors

Change in electrical resistance is the signalsent to direct-acting indicator (ohmmeter) or transducer.

B. Thermister

Voltage difference V is signal sent to direct-acting indicator (voltmeter) or transducer.

ReferenceJunction

MeasuringJunction

V

A. Thermocouple

Metal A

Metal B

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Because the equipment used to measure these parameters is itself electrical in nature, it is difficult to make clear distinctions as to what part of the equipment is the sensor and what part is the indicator. In fact, the measurements of volts, amperes, and ohms are all made with the same unit, the D’Arsonval meter (Figure 3-5). Electric current passing through the meter’s coil creates a magnetic field. The field reacts with the field of the permanent magnet surrounding it, causing the coil and the attached indicator needle to move.

A D’Arsonval meter connected into an electrical circuit can be made to measure electrical parameters without any special sensor. Which parameter is measured—volts, amperes, or ohms—depends on how the meter is connected in the circuit, as shown in Figure 3-6. Depending on what it measures, the D’Arsonval meter circuit will be referred to as a voltmeter, an ammeter, or an ohmmeter. The D’Arsonval meter is now rapidly being replaced by digital equipment that indicates the electrical value directly.

To measure power (watts), an instrument must combine the measure-ments of volts and amperes, because watts = volts × amperes (assuming the power factor is 1). The electrodynamic meter is used for this purpose. The electrodynamic meter is similar to a D’Arsonval meter, except that the permanent magnet in the D’Arsonval meter is replaced with a fixed coil. The interaction between the magnetic fields of the fixed coil and the coil attached to the indicator needle causes the needle to move.

FIGURE 3-5 D’Arsonval meter

Leech, D.P., Basic Electric Circuits, 3rd ed.,1984. Reprinted by permission of Prentice-Hall, Inc., Englewood Cliffs, NJ

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The electric energy used by a utility, which determines the utility’s electric bill, is measured in units of kilowatt-hours (kW�h). One thousand watts (that is, 1 kW) drawn by a circuit for 1 hour results in an energy consumption of 1 kW�h. The meters used to determine kilowatt-hour usage are essentially totalizing wattmeters (Figure 3-7).

FIGURE 3-6 Circuits used to measure volts, amps, and ohms by D’Arsonval meter movement

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On most kilowatt-hour meters, a rotating disk can be seen through the glass front or through a small window in the front of the case. An operator can count the revolutions of the disk by watching for the black mark on the disk. The speed of the disk indicates the kilowatt load. Checking the disk speed with a stop watch is a quick and accurate way of determining kilowatts used over a short period of time. The following formula may be used:

Equipment Status MonitorsA number of operating conditions associated with major equipment

should be monitored. Vibration should be monitored in a plant process control system particularly for large, expensive equipment such as pumps and blowers. Excessive vibration can quickly cause significant damage to this equipment, particularly when the equipment is operated at speeds greater then 1,800 rpm. Vibration sensors are available that, when mechanically attached to the particular piece of equipment, will activate a contact closure when vibration levels exceed a specific “g” value.

FIGURE 3-7 Totalizing wattmeter

kilowatts (disk watt-hours constant revolutions 3,600)××(seconds 1,000)×

-------------------------------------------------------------------------------------------------------------------------------------------=

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To minimize problems associated with adapting equipment to accommo-date these sensors in the field, vibration sensors should, if possible, be listed in the specifications as accessories to be included with equipment.

Position and speed are two other equipment status conditions that can be monitored. Position transmitters normally work with variable-resistance devices mechanically linked to a piece of equipment. The output of the variable-resistance device is then converted to a signal for monitoring. Speed transmitters are usually tachometers driven by the equipment being monitored. The tachometers produce a voltage that is converted to standard current values for monitoring purposes. Figure 3-8illustrates the construction and circuitry of a tachometer generator. The instrument uses an electrical coil that produces a voltage when excited by magnets mounted on a spinning shaft. This voltage is used to operate a milliammeter that provides an indication that is proportional to speed.

With some equipment, particularly clarifier drives, torque is another variable that is sometimes monitored. Torque sensing is normally used to shut down circuits to prevent damage to the equipment. Torque-sensing equipment can be supplied to produce a contact closure when torque rises above a preset value. It can also be supplied with converters that produce a 4–20-mA DC signal proportional to the magnitude of the torque so that the reading can power an indicator at a remote location.

Larger pumps and motors incorporate discrete sensors that measure parameters such as temperature, pressure, and position. These sensors are installed for safety monitoring and sequential control operations.

Process AnalyzersBecause of increasingly stringent requirements for water quality,

analytical equipment that measures process variables is used more

FIGURE 3-8 Construction and circuitry of tachometer generator

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frequently in water systems. Although such analyzers are not strictly a part of the distribution system, a brief review of such devices will complete the discussion of primary instrumentation.

Turbidity MonitorsTurbidity monitoring is now considered almost a must for the effluent

from granular-media filters. Turbidity monitoring can be most helpful in optimizing plant performance. The turbidity of raw water, as well as that of settled water and finished water leaving the treatment plant, is frequently measured. These measurements show water quality improve-ment at different stages in the treatment process.

Early turbidity-monitoring equipment used a sensor to detect changes in the amount of light transmitted directly through a sample. This concept of sensing was found to be nonresponsive when very low turbidity levels were being measured. Currently, turbidity-sensing devices monitor the intensity of light scattered from turbidity particles when a beam of light shines on the sample. The most common method measures the amount of light scattered in a direction at 90° to the path of the light beam.

At higher turbidity levels, this concept is not practical because the scattered light at high levels of turbidity cannot be measured accurately. One successful method of measuring the higher turbidity levels normally encountered in raw water is the surface scatter concept. Light scattered from turbidity particles at the surface of a sample compartment is measured to provide an indication of the turbidity level.

pH MonitorsAnother analytical instrument that has been used for some time in

water treatment plant applications is the pH monitor. Readings of the pH level can be extremely useful in monitoring and controlling a treatment process. Chemical reactions affecting coagulation, corrosion, and softeningcan be predicted, monitored, and changed through proper pH control. The pH monitors detect the level of hydrogen ion activity in a sample and convert it to a signal. A pH-sensing system consists of a glass electrode and a reference electrode. The glass electrode develops an electrical potential that varies with the pH of the process fluid. The potential developed between the two electrodes is amplified and converted to a signal representative of the pH.

Residual Chlorine MonitorsResidual chlorine monitoring is often required for a treatment

process. Two types of residual-chlorine-monitoring systems are used. One uses a chlorine-permeable membrane probe that allows chlorine to

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diffuse through a membrane system on the end of the probe. The chlorine concentration passing through the membrane generates a current in an electrode system that corresponds to the chlorine level. The other type of analyzer is the amperometric type. Two dissimilar metals are placed in a measurement cell containing an electrolyte. A voltage is applied to these two metal electrodes, and the amount of current flowing between the electrodes is proportional to the amount of chlorine present in the solution.

Particle CountersParticle counters are somewhat similar to turbidity monitors.

However, they provide a more detailed analysis of suspended matter in the sample. Particle counters transmit a laser beam across a sample stream, and the beam is detected by a sensor on the opposite side of the sample. When particles pass through the beam, they interrupt the beam. The sensor also senses particle size based on the amount of the beam that is interrupted. Particle counters can be programmed to provide information on particle count, total particle count below a specified particlesize, and particle distribution. The primary applications for these devices has been to monitor filter effluent in water treatment plants. However, particle counters may also monitor the effectiveness of coagulation and flocculation.

Streaming Current MonitorsThe streaming current analyzer monitors the efficiency of coagulation

in the turbidity-removal stage of a water treatment plant. The analyzer measures the particle charge remaining after coagulants have been added.

Secondary Instrumentation and TelemeteringSecondary instrumentation displays the signals from the sensors of

primary instrumentation. It also allows distant control. Secondary instru-ments are usually panel mounted. They can be mounted in local control panels, filter control consoles, area control panels, or main control panels.

Before World War II, most measuring and control instruments were mounted adjacent to the process being controlled, with direct connections to the process. These primary measuring devices required operators to move throughout the plant to take readings and make control adjustments. With a small process plant and an abundant workforce, this was sufficient.

Then, as plant monitoring, maintenance, and compliance require-ments increased, monitoring and control functions needed to be more centralized. Direct connection was no longer possible, so secondary instrumentation was developed. Secondary instrumentation measures the parameters and transmits signals that correspond to the measurements.

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Secondary instrumentation required the development of a signal trans-mission method, as well as field and panel-mounted hardware to perform the monitoring and control functions.

Pneumatic and Electronic Signal TransmissionThe first signal transmission methods used air pressure (i.e., pneu-

matic) transmission. Electronic signal transmission, developed later, uses electrical current or voltage signals. Both methods perform the same functions, and the instrument exteriors look more or less identical, both in the field and in the control room. As development and usage have progressed, both systems have also standardized signal levels. The pneu-matic standard range is 3 to 15 psig (2–100 kPa[gauge]), whereas the electronic standard is 4–20 mA DC. The practical advantage of this stan-dardization is that control room instruments’ operating mechanisms are based on common signal units in all locations. That is, the panel may contain indicators, recorders, and controllers handling pressure, tempera-tures, flows, and other process variables, but the input and output signals operate over the same standard range. The only differences among them are the display scales or charts employed. Figure 3-9 illustrates a trans-mitter with a remote receiver.

Pneumatic and electronic signal transmission systems can also be usedwithin the same process control system, allowing a great deal of flexibility in providing instrumentation for a plant. Equipment can be chosen that most effectively suits the application and environment. Signal converters are readily available to convert pneumatic pressure to electric current (P/I converters) and electric current to pneumatic pressure (I/P converters).

FIGURE 3-9 Pressure sensor connected to a remote indicator/recorder

Pressure SensorMounted in a Pipeline

Wire Connection

Indicator and Circular Chart Recorder

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Receivers and IndicatorsReceivers convert the signal sent by the transmitter to an indicator

reading for the operator to monitor. The indicators may be

• a direct-reading display that shows the current value of the parameter being monitored

• a recorder that preserves the information for later examination

• a totalizer that gives the total accumulated value since the instrument was last reset

• some combination of these units

Indicator displays and recorders are of two types: analog and digital. These terms are commonly used in describing various components and functions of instrumentation. Analog values range smoothly from the minimum to the maximum value of a given range. An analog signal is either a variable voltage or current. The dial indicator shown in Figure 3-10is an example of an analog display. Analog indicators include dial gauges and strip or circle charts (Figure 3-11). The indicated values on an analog display range smoothly from the display’s zero to its maximum. The D’Arsonval meter is commonly used as an analog indicator for electrical signals at standard voltage or current levels.

Digital values, on the other hand, take on only a fixed number of values within a range. Digital indicators, like digital watches, display decimal numbers. The number of possible readings within a given range is limited by the number of digits displayed. A mechanical digital indica-tor display is illustrated in Figure 3-12 and an electronic digital display in Figure 3-13.

Most parameters measured in water distribution are continuous in nature, like an analog signal or display. However, analog-to-digital converters allow continuous values to be displayed on digital indicators

FIGURE 3-10 Analog indicator display

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FIGURE 3-11 Typical circular chart recorder

Courtesy of Schlumberger Industries Water Division

FIGURE 3-12 Mechanical digital indicator display

FIGURE 3-13 Instrument with an electronic digital display

Courtesy of ATI Orion

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or transmitted over digital transmission channels. And digital-to-analog converters allow the reverse conversion.

Digital indicators are usually more accurate than analog indicators. They are not subject to the errors associated with electromechanical or mechanical systems, and they are easier to read correctly. However, analog indicators may be preferable for at-a-glance monitoring to ensure a value remains within a given range or to observe its rate of change.

TelemetryTo monitor conditions at very distant locations, such as a remote

pump station or reservoir, a telemetry system may be used. With this system, a sensor is connected to a transmitter, which sends a signal over a transmission channel to a combination receiver and indicator.

The type of signal used must be designed to maintain its accuracy over a long distance. Older equipment used audio tones or electrical pulses, but most equipment now transmits the information by a digital signal. The signal is transmitted either through direct wiring, through a leased telephone line, or by radio or microwave transmission. The receiver converts the signal to operate the indicator.

Telemetry systems allow flow rate, pressure, and other distribution system parameters to be sensed at one or more remote sites and indicated at a central location. Every telemetry system has the following three basic components:

• transmitter

• transmission channel

• receiver

The transmitter takes in data from one or more sensors at the remote site (Figure 3-14). It converts the data to a signal that is sent to the receiver over the transmission channel. The receiver changes the signal into standard electric values that are used to drive indicators and displays, recorders, or automatic control systems.

Telemetry Transmission ChannelsThe transmission channel in a telemetry system may be a water-utility-

owned cable for short distances, such as between two buildings on a common site. In most cases, however, the channel is either a leased tele-phone line, a radio channel, or a microwave system. A system using space satellites is also available, though it is expensive. The leased telephone line may be a dedicated metallic pair, which is relatively expensive but highly reliable and interference-free, or it may be a standard voice-grade phone line. Most modern transmitters generate signals that are designed

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to be sent over voice-grade lines. Fiber-optic cables are also increasingly being used.

Radio channels can be in the VHF (very high frequency) or UHF (ultrahigh frequency) band. Both radio and microwave systems generally require a line-of-sight path, which is unobstructed by buildings or hills between the transmitter and the receiver. To bypass obstructions or to ensure signal strength over very long distances, relay stations may be required.

Analog Signal SystemsAnalog signals that are commonly used are as follows:

• current—the DC current generated by the transmitter is proportional to the measured parameter.

• voltage—the DC voltage generated by the transmitter is proportional to the measured parameter.

• pulse-duration modulation (PDM)—the time period that a signal pulse is on is proportional to the value of the measured parameter.

• variable frequency—the frequency of the signal varies with the measured parameter.

Current and voltage signals can be used only for short-distance systems with utility-owned cable or a leased metallic pair. The signals can be damaged by line loss and other factors over telephone lines. Pulse-

FIGURE 3-14 Relay station for radio or microwave telemetry system

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duration modulation and variable-frequency signals can be used for any distance over any type of channel.

Digital Signal SystemsDigital systems generally send binary code, in which the transmitter

generates a series of on-off pulses that represent the exact numerical value of the measured parameter (for example, off-on-off-on represents 5).These signals can be used over long or short distances with any transmis-sion channel. The binary code signal is well adapted for connection to computerized systems. Figure 3-15 illustrates analog and digital telemetrysignals.

In digital systems, the remote or transmit device is normally referred to as a remote terminal unit (RTU) and the receiver is known as the control terminal unit (CTU). A rather significant difference between analog and digital telemetry systems is that in digital systems, the RTU does not itself directly measure the variables in the system. Instead, the variables are measured by a transducer device, which normally converts the physical value of the process variable into a current output signal of 4–20 mA DC. These signals in turn serve as input to the RTU. The RTU converts this data into a message consisting of digital words that are transmitted to the CTU.

MultiplexingMultiplexing systems allow a single physical channel, such as a

single phone line or radio frequency, to carry several signals simulta-neously. Tone-frequency multiplexing accomplishes this by having tone-frequency generators in the transmitter and tone-frequency filters in the receiver. The signal representing each measured parameter is assigned a separate audio frequency. The transmitter sends data representing each signal only over its assigned frequency, and the filters in the receiver allow it to respond to each frequency separately. Up to 21 distinct frequencies can be sent over a single voice-grade line. Tone-frequency multiplexing can be used with PDM and digital systems.

ScanningScanning is a second method of sending multiple signals over a single

line or transmission channel. A scanner at the transmitter end checks and transmits the value of each of several parameters one at a time, in a set order. The receiver decodes the signal and displays each value in turn. Scanning can be used with all types of signals and with all types of trans-mission channels. Scanning and tone-frequency multiplexing can be combined to allow even more signals over a single line. A four-signal

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scanner combined with a 21-channel tone frequency multiplexer would yield 84 distinct signal channels on a single line.

PollingAnother method of using a single line or channel to send several

different signals is known as polling. In a polling system, each instrument has a unique address, or identifying number. A system controller unit sends messages over the line telling the instrument at a given address to transmit its data. The process of asking an instrument to send data is called polling.

In many systems, the controller is programmed to poll instruments as often as necessary to monitor the system-some instruments may need checking more often than others. In more sophisticated systems, the controller regularly scans the status of each instrument to see whether there is new information to be transmitted. If the status indicates that new information exists, the controller instructs the instrument to send its data. In some systems, critical instruments also have the capability to interrupt a long transmission by another instrument in order to call the controller’s attention to urgent new data. Since data is transmitted only when needed

FIGURE 3-15 Telemetry signals— analog or digital form

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under the polling system, a single line or channel can handle more instru-ments than with a simple scanning system.

DuplexingIn many telemetry installations, the instrument signals received at the

operator’s central location may require the operator to send control signals back to the remote site. Duplexing allows this to be accomplished with a single line in three different ways:

• Full duplex allows signals to pass in both directions simultaneously.

• Half duplex allows signals to pass in both directions, but only in one direction at a time.

• Simplex allows signals to pass in only a single direction.

Full-duplex systems usually make use of tone-frequency generators to divide the line into transmission and receiving channels. Half-duplex systems may use tone-frequency generators, or they may simple rely on timing signals (similar to scanners) or on signals indicating status, such as end-of-transmission or ready-to-receive.

Control SystemsControl systems consist of three distinct components: signal condi-

tioners, actuators, and control elements.

• Signal conditioners receive either pneumatic, electric, or electronic signals from a controller. These signals are then conditioned or amplified, and used to initiate the actuator. Signal conditioners include solenoids, starters, and positioners.

• Actuators produce either rotary or linear movement of the final element. Actuators are usually motors or hydraulic cylinders and their related gearing (e.g., valve operators and motor controllers). These controls were discussed in the modules on valves and motors.

• Final elements are equipment such as pumps and valves that change the process fluid.

Understanding the relationship among these three elements is important. In many cases, the elements are supplied by different manu-facturers, and each component must meet the system requirements. The

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combination of these three will produce a final control element with its own distinct characteristics.

Two different types of controls are required in a process control system: two-state or continuous. Two-state control requires the final control element to be either on (open) or off (closed). Continuous control (also called modulation control) requires the element to vary its operation between the minimum and maximum points. An example is a valve opera-tor, which may be designed to (1) operate a valve either fully opened or closed, or (2) throttle the valve at intermediate positions.

In addition to the two types of control and the three components of final control elements, a variety of control media (air, electric, hydraulic) and several actuator types are available.

Control ClassificationsControl equipment can be completely independent of instrumentation,

or it may operate in direct response to instrument signals. The principal classifications of control are

• direct manual

• remote manual

• semiautomatic

• automatic

Direct Manual ControlIn a direct manual control system, the operator directly operates

switches or levers to turn the equipment on or off or otherwise change its operating condition. A valve operated by a handwheel is a common manually controlled piece of equipment. Operating electrical equipment requires throwing levers on the motor starter. Manual control has the advantages of low initial cost and no auxiliary equipment that must be maintained, but equipment operation may be time-consuming and laborious for the operator.

Remote Manual ControlWith remote manual control, the operator is also required to turn a

switch or push a button to operate equipment. However, the operator’s controls may be located some distance from the equipment itself. When the operator activates the control switch, an electric relay, solenoid, or motor is energized, which in turn activates the equipment. Power valve operators and magnetic motor starters are common examples of remote manual control devices.

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The solenoids and relays used for remote control are common components of all types of control systems. A solenoid (Figure 3-16) is an electric coil with a moveable magnetic core. When an electric current is passed through the coil, a magnetic field is generated that pulls the core into the coil. The core can be attached to any piece of mechanical equipment that needs to be moved by remote control. Solenoid valves (Figure 3-17) are very common in water system controls.

A relay is constructed of a solenoid that operates an electrical switch or a bank of switches. The most common use of a relay is to allow a rela-tively low-voltage, low-current control circuit to activate a high-voltage, high-current power circuit. A typical power relay and a control relay are illustrated in Figure 3-18.

FIGURE 3-16 Operation of a solenoid

FIGURE 3-17 Solenoid valves

Photo courtesy of ASCO

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Semiautomatic ControlSemiautomatic control combines manual or remote manual control

with automatic control functions for a single piece of equipment. A circuit breaker, for example, may disconnect automatically in response to an overload, then require manual reset.

Automatic ControlAn automatic control system turns equipment on and off or adjusts

its operating status in response to signals from instruments and sensors. The operator does not have to touch the controls under normal condi-tions. Automatic control systems are quite common. Simple examples are a thermostat used to control a heating system and automatic activation of lighting systems at night.

A number of modes (logic patterns) of operation are available under automatic control. Two common modes are on-off differential control and proportional control.

• On-off differential control is used to turn equipment full on when a sensor indicates a preset value, then turn it full off when the sensor indicates a second preset value. Figure 3-19illustrates this method of control, showing a chlorinator that is turned on and off at the same time as a service pump.

FIGURE 3-18 Typical relays

Eagle Signal Controls

Power Relay Control Relay

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• Proportional control is used to open a valve or increase a motor’s speed when the sensor shows a variation from a preset intended value. A common application is control of a chemical feeder in response to a flowmeter or residual analyzer signal.

Automatic controllers attempt to imitate human decision making, but they cannot achieve the level of complexity in decision making that humans can. Therefore, automatic control is limited to the more simple process situations. However, many water utility processes are in this category. The various pieces of equipment used to control each process parameter form a control loop of information processing. The direction of the information flow loop can be in either the same direction as the process (feedforward control) or the opposite direction of the process (feedback control).

A feedforward control loop measures one or more inputs of a process, calculates the required value of the other inputs, and then adjusts the other inputs to make the correction. Figure 3-20 illustrates this method of control, for which a chlorinator feed rate is automatically controlled (paced) in response to a signal from a flow transmitter.

Since feedforward control requires the ability to predict the output, this type of control is sometimes called predictive control. Furthermore, since feedforward control does not measure or verify that the result of the adjustment is correct, it is also referred to as open-loop control. A conse-quence of open-loop control is that if the measurements, calculations, or

FIGURE 3-19 On-off differential control of a chlorinator

Feed Rate Adjuster

Chlorine Gas

Chlorine Solution

SolenoidValve

WaterSupply

Pump Motor Starter

Treated Flow

Service Pump

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adjustments are wrong, the control loop cannot correct itself. In the exam-ple in Figure 3-20, there is no check built into the control system that the treated flow chlorine residual is actually at the desired concentration.

A feedback control loop measures the output of the process, reacts to an error in the process, and then adjusts an input to make the correction. Thus, the information loop goes backward. Figure 3-21 illustrates the control of a chlorinator in response to a residual analyzer. The analyzer continuously adjusts the feed rate of the chlorinator to provide the desired residual in the treated flow. It automatically adjusts for changes in flow rate or chlorine demand.

FIGURE 3-20 Feedforward control

FIGURE 3-21 Feedback control

Feed Rate Adjuster

Chlorine Gas

Chlorine SolutionWater Supply

Control Signal

Treated Flow

Flow Transmitter

Meter

Chlorine Feeder

Chlorine Gas

Water Supply

Treated Flow

Residual Analyzer–Controller

Residual Control

Signal

Chlorinated Water SampleChlorine Solution

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Since the process only reacts to an error, it is also called reactive control. Furthermore, since feedback control checks the results of the adjustment, it is said to be closed-loop control. Thus, feedback control, unlike feedforward control, is self-correcting. If the initial adjustment in response to changed conditions does not produce the correct output, the closed-loop system can detect the problem and make another adjustment. This process can be repeated as often as necessary until the output is correct.

Direct Wire and Supervisory ControlWithin a plant or an attended pump station, equipment is usually

connected to the central control panel through electric wiring. This approach is known as direct-wire control. When a remote station is unat-tended, the station equipment is controlled from an operator’s control location by supervisory control equipment, which transmits control signals over telemetry channels. The unattended site is sometimes called the outlying station, and the operator’s station may be called the dispatch station. Many large utilities have a large, central facility to monitor and control the entire water treatment plant (Figure 3-22).

FIGURE 3-22 Water treatment plant control center

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Instrumentation Operation and Maintenance

SCADA System ComponentsSupervisory control and data acquisition (SCADA) is a relatively new

tool for controlling and monitoring water facilities. SCADA combines many of the elements already discussed in this module, such as RTUs and control elements.

In a SCADA system, the control can be remote or automatic. SCADA subsystems consist of

• remote terminal units (RTUs)

• communications (telemetry transmission)

• a master station

• human-machine interface (primarily video display screens)

The first two subsystems have already been discussed in this module.

Master StationThe main function of a master station includes scanning the RTUs,

processing the data, transmitting operator commands, and maintaining a database of historical data (such as valve positions, flow, and pressure).

A master station consisting of a single computer is called a centralizedstation. A distributed system consists of several computer control devices.

Centralized computer control. The earliest SCADA systems consisted of a single master station controlling several RTUs, as illustrated in Figure 3-23. A problem with this system is its dependency on a single computer instal-lation and the communications links to the equipment. For instance, if a remote pumping station is completely operated by a central computer, loss of the computer or the telephone line to the station could mean complete loss of use of the station.

Distributed computer control. As smaller, more powerful, and less expensive computers have been developed, it has become possible to integrate computer control for subsystems of RTUs as well as for individual pieces of equipment. Figure 3-24 illustrates a distributed SCADA system. In the example, each remote pumping station can now be operated by its own computer, so that it continues to function properly in spite of the loss of the communications link with the central computer. This trend is now extending to increasing numbers of “smart equipment” that adjust themselves and monitor their own operation. This reduces the load on the central computer and decreases the system’s dependency on that computer for operation.

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FIGURE 3-23 Centralized SCADA system

Source: Keiji et al. (1994).

FIGURE 3-24 Distributed SCADA system controlling three remote pumping stations

Source: Keiji et al. (1994).

Human--MachineInterface

Human--MachineInterface

RemoteStation 1SCADA

Processor

RemoteStation 2SCADA

Processor

RemoteStation 3SCADA

Processor

HistoricalData

Processor

ModelProcessor

OtherProcessors

RTU RTU RTU RTU RTU RTU

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Human-Machine InterfaceThis interface is the connection between the operator and the computer

or SCADA system, usually by video display screen, keyboard, and mouse. Current SCADA systems allow personnel with no programming knowledge to set up the display. Operators can design graphics and tabular displays that precisely meet their needs. These displays may be interactive. For example, the symbol for a pump may change color or shape dependingon status, or a reservoir icon may “fill” as the reservoir level increases. Figure 3-25 shows a typical computer control center.

The Future of Supervisory ControlFew water systems exactly fit into any one category of control system.

Many very small systems are still primarily operated manually, but there are now a few that are almost completely automated. Most water systems, though, are somewhere in between. It is clear that the future will bring increasing automation.

State regulatory agencies have mixed emotions about allowing complete automation of water systems. In some ways, automation may be better than manual control. It is usually more precise, and it eliminates the possibility of human error that is always possible with manual control. On the other hand, although computers and computer programs are becoming increasingly reliable, a simple “bug” in the computer

FIGURE 3-25 Central computer command center

Courtesy of ABB, Inc.

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program could seriously damage equipment or cause contamination of the water supply. State regulators will always want to make sure that every automated system is supplied with all possible auxiliary monitoring. The system must be positively designed to shut down or take other appropriate action if anything goes wrong.

One of the newer innovations in computer control is the expert system, which is an interactive computer program that incorporates judgment, experience, rule of thumb, intuition, and other expertise to provide advice about a variety of tasks. In short, the computer is programmed to attempt to copy the decision-making process that would be made by a human expert.

Such systems are still in the early stage of development and are extremely complex. However, they are seen as eventually having a place in water utility operation by providing sophisticated control over certain operations, resulting in safer and more economical operation.

Instrumentation MaintenanceInstrumentation represents about 8 percent of the capital investment

in mechanical equipment for the average distribution system. Although this may not be a large sum, the proper operation of controls can make a substantial contribution to cost savings by improving overall system performance.

Most new equipment is much more reliable and requires less mainte-nance then instrumentation of just a few years ago. There are, however, numerous routine maintenance tasks that an operator can perform to keep the instrumentation functioning properly. An instrument’s useful life can be increased significantly if it is routinely checked to be sure it is not exposed to moisture, chemical gas or dust, excessive heat, vibration, or other damaging environmental factors.

Pressure SensorsEvery sensor that in any way responds to liquid pressure will

perform poorly if air enters the sensor. Air or gas may enter into the sensing element when released from the fluid line. A vent should be provided on the mechanism and used on a regular basis to allow any accumulated air to escape. A maintenance schedule can eventually be developed as operators gain experience working with the particular sensors. Wherever possible, the sensor should be installed so that trapped gasses flow into the upper part of the monitored line, rather than into the sensor.

In cold climates, freezing is a possibility. Most pressure sensors either have provision for a heater and thermostat or can be equipped with

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special protective cabinets. Heater tape can also be used for protecting sensing lines.

The small pipe connections used to connect pressure sensors, switches, and gauges to the pipeline can be a continual source of trouble where dissimilar metals (for instance, cast iron and brass or bronze) are used together. A valve cock should be installed between the pipeline tap and the device to allow easy disconnection and service. At least twice a year, the sensor should be removed and the valve cock blown and rodded out if necessary. Corrosion of nipples should be checked, and if any weak-ness is apparent, the failed part should be replaced.

FlowmetersSeveral types of flowmeters are used in distribution systems, and

each has its own particular maintenance requirements. A venturi tube, orifice, or flow nozzle type of meter has small ports that connect the process fluid to the transmitter mechanisms. These parts should be blown out periodically.

Propeller meters are all-mechanical devices. Over a period of time, wear produces lower readings. The manufacturer’s recommendations for lubrication should be carried out regularly, and only the recommended lubricant should be used. A magnetic meter is an electrical unit, so regular checks should be made for corrosion or insulation breakdown around conduit connections or grounding straps.

TransmittersThe operator with no special training in transmitter repair can

generally do little more in the way of maintenance than to ensure a favorable environment for the equipment. In hot climates, it is important to protect electronic transmitters from exposure to high temperatures. Most electronic transmitters are rated for operation up to 130°F (55°C). Beyond this temperature, many electronic components may break down. Direct exposure to sunlight may contribute to serious heat buildup within units, and it is often necessary to provide a shield or fan if components are confined in a cabinet. Most transmitters are splash-proof and will function with occasional exposure to wetting. However, unless otherwise specified, most units will not withstand being submerged in water. Therefore, metering pits should have pumps and level alarms to warn of flooding.

Receivers and IndicatorsReceiving and indicating devices usually require special skills for

service and maintenance. However, the operator should attempt to maintain a favorable environment for the equipment. A dirty or damp

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atmosphere may damage receivers. Vibration, chemical dust (such as fluoride), and high chlorine content in the atmosphere should all be avoided. The high temperatures that affect transmitters also cause problems for receivers, and inking systems on recording indicators may perform poorly in the cold.

Troubleshooting GuidelinesSome general guidelines for troubleshooting instrumentation

systems are as follows:

• Never enter the facility with tools of any type—most of us have a tendency to start taking things apart without a thorough diagnosis.

• Make a complete diagnosis, and attempt to confirm it by example.

• Consider ways to deal with the safety risks.

• Consider what will happen to the operating system while maintenance is being performed. In particular, where control valves are involved, consider the possibility of water hammer.

• Always inform the proper authorities of planned actions beforehand.

Maintenance RecordsThe development of a maintenance file on instrumentation has many

long-term benefits for any distribution system, including the following:

• accurate accounting of the cost

• indication of areas where there may be a need for a change in the type of equipment

• projection of the life expectancy of each type of equipment

• knowledge required to maintain parts inventories at minimum cost (based on operating experience)

• proper parts inventories that can reduce emergency downtime substantially (another form of cost savings)

• guidance for preventive maintenance steps

Many facilities today have computers that print out maintenance schedules for each type of equipment as a guide to the preventive measures that need to be performed (Figure 3-26). Where computers are

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not used, reminders can be placed in an instrument chart supply so that the reminder comes up as the charts are rotated.

Maintenance records should include some form of index card for each instrument. One of the most common problems for maintenance program continuity is the loss of instruction manuals and calibration sheets. A file should be provided for these items, and care should be taken to avoid losing them, especially when responsible personnel are transferred or terminated.

Records should also be kept on services related to instruments not owned by the utility, such as telephone lines and power supplies to pump stations. Such information should be carefully documented so that there is concrete evidence in case it becomes necessary to register a complaint. System diagrams showing the routings for communication links are important. For telemetry systems, it is useful to have a map showing channels or frequencies in each tone spectrum and the facilities to which they are connected.

■ ■■

FIGURE 3-26 Computer printout of maintenance schedule